Methods and systems for measuring anions

ABSTRACT

Embodiments of the present disclosure provide for methods for detecting the presence and/or concentration of anions in a solution, systems for detecting the presence and/or concentration of anions in a solution, anion sensor systems, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/041,098, filed on Feb. 11, 2016, which claims the benefit of andpriority to U.S. Provisional Application Ser. No. 62/115,354, having thetitle “Methods and Systems for Measuring Anions,” filed on Feb. 12,2015, the disclosure of which is incorporated herein in by reference inits entirety.

BACKGROUND

Anions play a pivotal role in biological, chemical and environmentalprocesses, and they have begun to receive increased attention. Amonganions, iodide is of special note not only because of its physiologicalimportance in controlling metabolic activities but also as an essentialfactor in energy conversion processes. Both deficiency and excess ofiodide can result in malfunction and disorders in the human body. Inaddition to its environmental and biological importance, iodide is a keycomponent of a redox couple often used in solar-energy-harvestingsystems. Various methods can be used for the determination of iodide,including photoluminescence, colorimetric detection, time-resolvedabsorption techniques, mass spectrometry, chromatography, Ramanscattering and electrochemical profiling. However, because of its largesize and weakly basic nature, the binding capacity of iodide is theweakest among the halide ions. Therefore, the development of a simple,rapid, direct and economical method for the determination of iodide isstill under investigation.

Researchers have recently demonstrated high interest in the developmentof photoluminescence-based methods for the highly sensitive, rapid andselective determination of iodide. However, the complex preparation ofthe fluorophores, their low hydrophilicity, and the fact that themultistep determination of iodide usually involves a toxic reagent (Hg)has hampered the widespread application of photoluminescence-basedmethods.

SUMMARY

Embodiments of the present disclosure provide for methods for detectingthe presence and/or concentration of anions in a solution, systems fordetecting the presence and/or concentration of anions in a solution,anion sensor systems, and the like.

An embodiment of the present disclosure includes a method of detectinganions, including: mixing a solution including an anion with a Pt(II)porphyrin; irradiating the solution with visible light; measuring aphotoluminescence signal from the Pt(II) porphyrin; and determining thepresence of the anion. In an embodiment, the anion can be selected fromthe group consisting of: a halogen anion, a sulfide anion, and a cyanideanion.

An embodiment of the present disclosure includes a sensor system,including: a structure having a Pt(II) porphyrin disposed on thesurface; a compartment including a solution including an anion, whereinthe anion and Pt(II) porphyrin interact to quench the photoluminescenceof the Pt(II) porphyrin once the structure is placed in the compartmentwith the solution; a system or device for irradiating the solution withvisible light; and a system or device for measuring a signal selectedfrom a photoluminescence signal, UV-Vis signal, or a combinationthereof, wherein a change in the signal relative to a signal without theanion present is correlated to the concentration of the anion in thesolution.

An embodiment of the present disclosure includes a method, including:placing a structure having a Pt(II) porphyrin disposed on the surface ina compartment including a solution including an anion, wherein the anionand Pt(II) porphyrin interact to quench the photoluminescence of thePt(II) porphyrin; irradiating the solution with visible light; andmeasuring a signal selected from a photoluminescence signal, UV-Vissignal, or a combination thereof, wherein a change in the signalrelative to a signal without the anion present is correlated to theconcentration of the anion in the solution.

Other systems, methods, features, and advantages will be or becomeapparent to one with skill in the art upon examination of the followingdrawings and detailed description. It is intended that all suchadditional compositions, apparatus, methods, features and advantages beincluded within this description, be within the scope of the presentdisclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows absorption (left) and photoluminescence (right, λex=512 nm)spectra of Pt(II)TMPyP before and after the additions of chloride,bromide, and iodide in the aqueous phase.

FIG. 2 shows the transient absorption spectra of Pt(II)TMPyP before andafter successive additions of chloride, bromide and iodide in theaqueous phase (λ_(ex)=350 nm; λ_(monitoring)=350-900 nm), excitationenergy density of 2.17 mJ/cm².

FIG. 3 shows kinetic traces of Pt(II)TMPyP with and without thesuccessive addition of chloride, bromide and iodide in the aqueousphase. Monitoring wavelengths are 443 nm for excited-state absorptionand 401 nm for ground-state bleach.

FIG. 4 illustrates luminescence quenching of Pt(II)TMPyP due tophotoinduced reduction by iodide in the aqueous phase.

FIG. 5 plots the relation of photoluminescence intensity against theiodide added into Pt(II)TMPyP and linear fit for estimation of detectionlimit.

FIG. 6 demonstrates UV-Vis absorption (left), and photoluminescence(right, λ_(ex)=512 nm) spectra for Pt(II)TMPyP and after successiveadditions of various halides, i.e. chloride, bromide, and iodide inaqueous phase.

FIG. 7 shows Stern-Volmer plots for Pt(II)TMPyP, and with the maximumamounts of halides added into it for similar quenching (corresponding toPL spectra in FIG. 6).

FIG. 8 illustrates time correlated single photon counting forPt(II)TMPyP porphyrin, and with successive additions of iodide inaqueous phase. Corresponding lifetimes extracted from kinetic tracesmonitored at μ_(em)=660 nm are given on graph and standard errorsincluded in parenthesis.

FIG. 9 shows UV-Vis absorption (left), and photoluminescence (right,λ_(ex)=512 nm) spectra for Pt(II)TMPyP, and after successive additionsof iodide in organic phase, methanol.

FIG. 10 shows UV-Vis absorption (left), and photoluminescence (right,λ_(ex)=563 nm) spectra for Zn(II)TMPyP and after successive additions ofiodide in aqueous phase.

FIGS. 11A-B show transient absorption spectra (11A), and kinetics (11B)for Zn(II)TMPyP porphyrin, and upon its interaction with iodide inaqueous phase (λ_(ex)=350 nm; λ_(monitoring) 350-900 nm), excitationenergy density=0.36 mJ/cm². Monitoring wavelengths are 470 nm forexcited-state absorption and 440 nm for ground-state bleach.

FIG. 12 shows femtosecond transient absorption spectra of Pt(II)TMPyPporphyrin, and with addition of iodide in aqueous phase (λ_(ex)=350 nm;λ_(monitoring) 350-700 nm), excitation energy density=1.44 mJ/cm².

FIG. 13 shows the kinetics of femtosecond transient absorption spectraof Pt(II)TMPyP porphyrin, and with addition of iodide in aqueous phase.Monitoring wavelengths are 443 nm for excited-state absorption and 401nm for ground-state bleach.

FIG. 14 shows steady-state UV-Vis absorption (left), andphotoluminescence (right, λ_(ex)=512 nm) spectra for Pt(II)TMPyP, andafter successive additions of fluoride in aqueous phase.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present disclosure will be limited onlyby the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit (unlessthe context clearly dictates otherwise), between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of chemistry, synthetic organic chemistry, polymerchemistry, analytical chemistry, and the like, which are within theskill of the art. Such techniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the compositions and compounds disclosed andclaimed herein. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperature, etc.), but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C., and pressure is in bar.Standard temperature and pressure are defined as 0 ° C. and 1 bar.

Before the embodiments of the present disclosure are described indetail, it is to be understood that, unless otherwise indicated, thepresent disclosure is not limited to particular materials, reagents,reaction materials, manufacturing processes, or the like, as such canvary. It is also to be understood that the terminology used herein isfor purposes of describing particular embodiments only, and is notintended to be limiting. It is also possible in the present disclosurethat steps can be executed in different sequence where this is logicallypossible.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a support” includes a plurality of supports. In thisspecification and in the claims that follow, reference will be made to anumber of terms that shall be defined to have the following meaningsunless a contrary intention is apparent.

General Discussion

Embodiments of the present disclosure provide for methods for detectingthe presence and/or concentration of anions in a solution, systems fordetecting the presence and/or concentration of anions in a solution,anion sensor systems, and the like. Embodiments of the presentdisclosure can be used in the biomedical field, pharmaceutical field,food and beverage field, environmental field, solar harvesting systems,oil and gas field, petrochemical research, industrial field, and thelike, to determine the presence of an anion(s) and measure theconcentration of the anion(s).

Embodiments of the present disclosure provide for an easy and rapidmethod for ultrasensitive detection of low pico-mole (ppt; parts pertrillion) levels of anions such as iodide in solution such as in theaqueous phase. An embodiment of the present disclosure provides for anon-toxic porphyrin sensor for easy, rapid and economical detection oftrace levels of an anion such as iodide upon directing light onto thesolution. In particular, an embodiment provides the ability to generatephotoluminescence spectra and the linear change of intensity withaddition of iodide ions can be determined. Furthermore, studies wereperformed to understand the photo-physics of the phenomenon and itsrelation to the specific composition of the porphyrin. Additionaldetails and embodiments are provided in the following Examples.

An embodiment of the present disclosure includes methods that include asingle step direct technique for monitoring an anion(s) in a solution(e.g., aqueous phase) with picomole detections capabilities.

An embodiment of the present disclosure includes a method of detectinganions. In an embodiment, the anions can include a halogen anion (e.g.,iodide), a sulfide anion, and a cyanide anion. In general, the methodincludes mixing a solution including an anion with a Pt(II) porphyrin.In an embodiment, the Pt(II) porphyrin can include 5, 10, 15,20-tetra(1-methyl-4-pyridino)-porphyrin (Pt(II) tetrachloride, analoguesthereof, derivatives thereof, and the like. In an embodiment, the anionsand Pt(II) porphyin can be dissolved in a solution such as water,methanol, DMSO, acetonitrile, combinations thereof, and the like.

Subsequently, the solution can be irradiated with visible light. Thephotoluminescence signal from the Pt(II) porphyrin can be measured andthe photoluminescence signal can be correlated with the presence and/orconcentration of the anion. In an embodiment, a combination of anionscan be detected and the concentration measured. In an embodiment, aUV-Vis signal can be measured as well and correlated to the presence andconcentration of one or more anions.

In an embodiment, the method includes placing a structure (e.g., teststrip) having a Pt(II) porphyrin disposed on the surface into acompartment including a solution including an anion. Once the structureis placed into the compartment (e.g., cuvette, a test tube, beaker, orthe like), the anion and Pt(II) porphyrin interact to quench thephotoluminescence of the Pt(II) porphyrin. Next, the solution isirradiated with visible light. Thereafter, a signal is measured. In anembodiment, the signal can be a photoluminescence signal and/or a UV-Vissignal. A change in the signal relative to a signal without the anionpresent is correlated to the concentration of the anion in the solution.

More particularly, in the presence of an anion such as iodide, thequenching of the photoluminescence of the Pt(II) porphyin can bemonitored upon irradiation with visible light. This method does notrequire complex preparatory steps or additional species such as Hg,which is often used for the activation of fluorophores.

Ultrafast laser spectroscopy or similar spectroscopy or techniques canbe used to measure the quenching of the photoluminescence. Differentanions can quench to different magnitudes. The detection limit can bedetermined using the 3σ IUPAC method, where the detectionlimit=3σ/slope. In an embodiment, a calibration curve forphotoluminescence intensity against anion concentration can be used todetermine the concentration. The slope value was taken from the curve,and the standard deviation σ was calculated for the photoluminescenceintensity of the blank Pt(II)TMPyP solution in the absence of the anion.In this way, the concentration can be determined. In an embodiment, theanion can be detected at a level of about 20 to 50 pmol or about 30 pmolin the solution, while in some embodiments the anion can be detected tolevels down to about 20 pmol or down to about 30 pmol.

In addition to measuring the quenching of the photoluminescence, UV-Visspectra also illustrated a decrease in the optical absorbance. In thisway, each anion can have its own unique “signature” (e.g.,photoluminescence spectra and UV-Vis spectra) that can be used todifferentiate various anions. In an embodiment, calibration curves canbe constructed for different anions for the photoluminescence spectraand/or UV-Vis spectra and can be used to identify the anion(s) anddetermine the concentration of the anions in the solution.

An embodiment of the disclosure also includes a sensor system. In anembodiment, the sensor system includes a structure having a Pt(II)porphyrin disposed on the surface or alternatively, the Pt(II) porphyrincan be a solid or liquid that is mixed with the solution of interest. Inaddition, the sensor system includes a compartment including a solutionincluding an anion. The anion and Pt(II) porphyrin can interact toquench the photoluminescence of the Pt(II) porphyrin. The sensor systemcan include a system or device for irradiating the solution with visiblelight (e.g., an irradiation system). The sensor system can also includea system or device for measuring a signal (e.g., a photoluminescencesignal and UV-Vis signal). As described above and in the example, achange in the signal relative to a signal without the anion(s) presentcan be correlated to the concentration of the anion(s) in the solution.Additional details and embodiments are provided in the followingExamples.

In an embodiment, a Pt(II) porphyrin based photoluminescence sensor(e.g., a 5,10,15,20-tetra(1-methyl-4-pyridino)-porphyrin Pt(II)tetrachloride (Pt(II)TMPyP)-based photoluminescence sensor) was used forthe ultra-sensitive and rapid determination of an anion (e.g., iodide).An embodiment of the method allows for use of a one-step directtechnique for monitoring iodide in an aqueous phase with a pico-mole(pmol) detection limit. In a particular embodiment where the anion isiodide, iodide can be used to quench photoluminescence of Pt(II)TMPyPand can be directly monitored upon irradiation with visible light. Thesensor and method do not use hazardous materials like other sensors andtechniques while also provide exceptional detection limits in a shortamount of time.

EXAMPLES

Now having described the embodiments of the disclosure, in general, theexamples describe some additional embodiments. While embodiments of thepresent disclosure are described in connection with the example and thecorresponding text and figures, there is no intent to limit embodimentsof the disclosure to these descriptions. On the contrary, the intent isto cover all alternatives, modifications, and equivalents includedwithin the spirit and scope of embodiments of the present disclosure.

Example 1

In this Example, a simple method with a previously unattained (1×10⁻¹²M) detection limit in aqueous solution is proposed but also a detailedmechanism for photoluminescence quenching using cutting-edge ultrafastlaser spectroscopy with broadband capabilities is proposed.Interestingly, a control experiment using Zn(II)TMPyP clearly indicatedthat the Pt metal center is the element for the quenching and theextremely low detection limit.

Both of the investigated porphyrins, Pt(II)TMPyP and Zn(II)TMPyP, weresupplied by Frontier Scientific. High-purity halide salts of NH₄F, NaCl,KBr and Kl were purchased from Sigma-Aldrich. Porphyrins and halidesalts were completely dissolved in Milli-Q water, and all spectralmeasurements were performed without any additions. Absorption spectrawere recorded on a Cary 6000i UV-Vis-NIR spectrophotometer (Varian,Inc.). Steady-state photoluminescence spectra were recorded with aJobin-Yvon-Horiba Fluoromax-4 spectrofluorometer. Femto- and nanosecondtransient absorption spectroscopy was utilized to probe thephotophysical processes that occur upon the photoexcitation of thePt(II)TMPyP with and without iodide. The experimental setup is detailedelsewhere.⁴⁷⁻⁴⁹

FIG. 1 displays the steady-state absorption and photoluminescencespectra of Pt(II)TMPyP alone and with various halide ions, i.e.,chloride, bromide and iodide. Three milliliters of 12.8 μM Pt(II)TMPyPaqueous solution was added to a quartz cuvette with a 1 cm path length,and an aqueous solution of a halide ion was then added. Samples wereexcited at 512 nm, and the aperture size for both the entrance and exitslits was kept the same for all experiments. The obtained emissionagrees in shape and position to previously reported Pt(II) porphyrinemission which is mainly due to heavy atom effect of Pt(II) as centralatom leading to efficient intersystem crossing (ISC).^(50, 51) Asevident in FIG. 1 (left panel), the change in peak intensity andposition in the absorption spectrum of Pt(II)TMPyP after halide additionwas negligible. In contrast, a significant intensity quenching in thephotoluminescence spectrum upon halide ion addition is clearly evidentin the figure. The quenching magnitude for the different halidesdecreased in the order iodide>bromide>chloride. The addition of 1.2 μmolof chloride led to approximately 28% quenching of the photoluminescenceof Pt(II)TMPyP, and the extent of quenching increased down the halogengroup, with almost 75 and 90% quenching observed after the additions ofsimilar amounts of bromide (1.05 μmol) and iodide (1.07 μmol),respectively. It is worth mentioning that fluoride didn't show anysignificant tendency to interact with Pt(II)TMPyP (see FIG. 14).

The literature contains only a few reports on the ultra-trace,low-pmol-level detection of iodide. Wygladacz et al. reported afluorescent microsphere fiber optic microsensor array for the low-pmoldetection of iodide.²⁹ However, the microsensor exhibited the lowestlimit of detection of 3 pmol only at pH 3.5. Recently, Dasary et al.reported a surface-enhanced Raman scattering probe for ultra-sensitivedetection.⁴³ An iodide detection limit of 30 pmol was demonstrated onthe basis of indirect Raman intensity originating from the desorption ofrhodamine from gold nanoparticles.

The detection limit was determined from our photoluminescence quenchingresults using the 3σ IUPAC method, where the detectionlimit=3σ/slope.^(20, 24, 26, 33, 36) A calibration curve forphotolumine-scence intensity against iodide concentration was firstconstructed (FIG. 5). The slope value was taken from the curve, and thestandard deviation σ was calculated for the photoluminescence intensityof the blank Pt(II)TMPyP solution in the absence of iodide.Interestingly, a never-before-attained detection limit of 1 pmol wasobtained.

The photoluminescence quenching upon the addition of halides indicatesthe presence of a photoinduced electron transfer event between thehalide donor and the Pt(II)TMPyP acceptor. Stern-Volmer plots for all ofthe halide ions show a deviation from linearity with Pt(II)TMPyP (FIGS.6 and 7), indicating that a static reaction is involved in the quenchingprocess. The observed negative deviation from linearity in Stern-Volmerplots at very high halide concentration can be attributed to theformation of a luminescent exciplex between the negatively chargedhalide and the poor electron density on the Pt metallic center(explained below in transient measurements).⁵² From the relationshipK_(SV)=k_(q)τ^(o),⁵³ where K_(SV) is the Stern-Volmer constant, k_(q) isthe bimolecular quenching rate constant and τ^(o) is the phosphorescencelifetime of Pt(II)TMPyP (1.03 μs), we calculated k_(q)≈7.8×10¹² M⁻¹s⁻¹.This quenching rate far exceeds the diffusion-controlled limit (theestimated rate for iodide diffusion for an aqueous sample under thecurrent experimental conditions is ˜3×10¹⁰ M⁻¹s⁻¹, as determined usingthe Stokes-Einstein equation),⁵⁴ establishing the fact that thequenching is due to the static interaction between the donor-acceptorcomponents. To confirm the static quenching, the phosphorescencelifetimes of the Pt(II)TMPyP free and in the presence of differentiodide concentrations giving 30, 50, and 70% quenching were measured;the results are displayed in FIG. 8. The time decay curves collected forthe four solutions exhibited the same lifetime, confirming the staticnature of the quenching mechanism.⁵³ Further confirmation of the staticnature of the quenching mechanism is indicated by the similarity of theStern-Volmer quenching constants for samples in water and methanol (seeFIG. 9). Despite the different viscosities of water and methanol, theK_(SV) values extracted for the iodide interaction with Pt(II)TMPyP inmethanol is 8.6 μM⁻¹ and that in water is 8.01 μM⁻¹. The similaritybetween these values suggests that quenching does not depend on solventviscosity, which favors a static-interaction explanation.

Similar to the Pt(II)TMPyP and iodide system, 1.07 μmol of iodide wassequentially added to 3 mL of 12.7 μM Zn(II)TMPyP. The steady-state andexcited-state absorptions remained essentially the same before and afterthe iodide addition (FIGS. 10 and 11A-B). For the photoluminescencemeasurements of Zn(II)TMPyP with the addition of 1.07 μmol iodide, incontrast to the 90% photoluminescence quenching observed in theiodide-Pt(II)TMPyP mixture, only approximately 7% quenching was observed(FIG. 10). This quenching behavior demonstrates that the change in thephotoluminescence was specific to the metal center in the cavity of theporphyrin and pertained to its emissive state, as discussed below.

Transient absorption (TA) spectroscopy is an essential method forinvestigating photoinduced excited-state interactions.⁵⁵⁻⁵⁷ In thepresent work, the excited-state interaction of halides with Pt(II)TMPyPwas examined using nanosecond (ns) and femtosecond (fs) TA spectroscopy.The recorded ns-TA spectra of aqueous solutions of free Pt(II)TMPyP andits mixtures with chloride, bromide, and iodide collected after 350 nmphotoexcitation are shown in FIG. 2.

The ns-TA spectra for Pt(II)TMPyP with and without halides showground-state bleaching at ˜400 nm and excited-state T₁-T_(n) absorptionover the range 420-600 nm. These spectral features are consistent withthe reported excited-state triplet absorption for porphyrinmolecules.^(58, 59) Members of this class of porphyrins in which Pt isthe central atom are known to exhibit a very rapid and efficientsinglet-to-triplet (S₁→T₁) intersystem crossing associated with strongspin-orbit coupling.⁶⁰ The TA spectra for Pt(II)TMPyP show a change overa time window of up to 4 μs, which is in agreement with the reportedtriplet lifetime of aerated solutions (˜1 μs).⁵⁸ The TA spectra recordedin the presence of halides demonstrate a significant shortening of thetime window. Moreover, the time window over which TA spectra aredetected shows a further shortening in the same order ofchloride>bromide>iodide. The kinetic traces collected from the TAspectra at both the ground-state bleach recovery and the excited-stateabsorption decay are given in FIG. 3.

All kinetic traces both for ground-state bleach and excited-state decayare fitted to single exponent fitting curves as shown in the figure. Theground-state bleach recovery extracted from the TA spectra ofPt(II)TMPyP in the presence of the halides decreases in the order ofPt(II)TMPyP (1.03 μs)>Pt(II)TMPyP-chloride (0.56 μs)>Pt(II)TMPyP-bromide(0.09 μs)>Pt(II)TMPyP-iodide (0.04 μs). This overall fast deactivationof the triplet signal implies the presence of an extra process involvedin deactivation of the excited state when halides are present comparedto the case of free Pt(II) porphyrin. Notably, the ring reductionpotential of Pt(II) porphyrin to be in the range of −1.39 to −1.3,⁶¹whereas the oxidation potentials of the halides (X⁻/X) are 1.36, 1.087,and 0.535 for chloride, bromide, and iodide, respectively.⁶² The fastchanges in the TA spectra, the redox properties and the lack of anypossibility for energy transfer suggests a photoinduced electrontransfer from the halides to the Pt(II)TMPyP*. In general, a peripheralsubstituent at the meso position on a porphyrin macrocycle cansignificantly change its electrochemical potential. ⁶³

In this regime, the electron-accepting positively charged pyridiniumunits (CH₃-N⁺) on the meso positions of the porphyrin will extract theelectron density from the porphyrin macrocycle via intramolecular chargetransfer (ICT).⁶⁴ If we consider the ICT in conjunction with the Pt(II)as the central atom in the TMPyP core, Pt-to-porphyrin back electrondonating (d→e_(g)(π*))^(50, 65) produces a region of poor electrondensity on the Pt metallic center. This generation of this region isexpected to facilitate the axial electron transfer interaction with thePt central atom. Hence, upon photoexcitation, an attractive center isanticipated to be formed around the Pt in the center of the Pt(II)TMPyP,which, in turn, facilitates attraction of the negatively chargedhalides, triggering electron transfer. Notably, the strong spectraloverlap between the porphyrin anion radical, which is reported to occurat 470 nm, and T₁-T_(n) TA and the very low concentration of halide usedin these experiments make monitoring of the anion radical absorptiondifficult.⁶⁶

To confirm the importance of Pt as the central atom in the suggestedmechanism, a control experiment was carried out using Zn(II)TMPyP withiodide; in this experiment, the TA did not exhibit any change (see FIG.11A-B), confirming that the interaction occurs with the Pt at theporphyrin macrocycle center. Further confirmation of the fastphotoinduced electron transfer was provided by the fs-TA in FIGS. 12 and13.

In the case of Pt(II)TMPyP, the triplet-triplet absorption developedwithin 120 fs, which was the temporal resolution of our experiment. Thisresult is in good agreement with the efficient and fast ISC.^(50,51) Onthe contrary, the fs-TA for Pt(II)TMPyP in the presence of iodide (seeFIG. 13) exhibited a fast change in the ground-state bleach, indicatingthe rapid deactivation of the excited state because of photoinducedelectron transfer between iodide and Pt(II)TMPyP, as simplified in FIG.4. Taking advantage from this Pt(II)TMPyP-iodide system, furtherresearch is going on towards the development of porphyrin-based sensorsfor other crucial anions like sulfide and cyanide.

Conclusions

Many halogen containing compounds have come into everyday use in thefields of chemistry, biology, medicine, plastics, food and evenphotography. Many techniques for the detection of these halogencompounds and ions have been developed not only to reduce the complexityand cost of the analysis but also to improve the detection sensitivity.In these techniques various type of materials namely polymers,porphyrin, nano-particles/clusters, quantum dots, DNA logic gate, andcomposites have been tried as photoluminescence sensor for the detectionof halides.¹⁷⁻³² Requirement of controlled shape and size makesnano-sized materials less preferred choice as a sensor. Furthermore,deterioration and modification of nano-sized materials also questiontheir application. Herein, we report for the first time the photoinducedtriplet-state electron transfer of Pt(II)TMPyP as an easy, rapid,environmentally friendly, ultra-sensitive (a never-before-attaineddetection limit of 1×10⁻¹² M) and economical method for thedetermination of iodide in the aqueous phase. Pt(II) porphyrinphosphorescence was observed to be quenched to different magnitudesthrough the use of different halides. The efficiency of quenching wasexperimentally demonstrated to increase in the orderchloride<bromide<iodide. The very low concentration range over which thePt(II) porphyrin exhibit quenching and the simplicity of themeasurements constitute the basis for a very safe and efficient halidesensor in the analytical market.

Example 1 References

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Example 1 Supplemental Information

Estimation of Detection Limit for Iodide with Pt(II)TMPyP in AqueousPhase

$\begin{matrix}{{{Detection}\mspace{14mu} {Limit}} = {3\mspace{11mu} {\sigma/{slope}}}} \\{= {3 \times {0.025/67.54}}} \\{= {1\mspace{14mu} p\; {mol}}}\end{matrix}$

Multiple number of PL spectra (5-7) were recorded for the aqueous phasePt(II)TMPyP alone. Sample Standard Deviation was calculated for the peakintensity value using ‘Statistics on Columns’ option in origin softwareand verified with online calculator. Sample Standard Deviation (0.025)is close to Population Standard Deviation (0.022). Standard deviationfor the blank fluorophore, Pt(II)TMPyP porphyrin solution, without theaddition of iodide was 0.025.

In the presence of halide ions, the quenching of the photoluminescenceof Pt(II)TMPyP can be directly monitored upon irradiation with visiblelight. Pt(II) porphyrin phosphorescence was quenched to differentmagnitudes through the use of different halides (FIG. 1). The efficiencyof quenching was experimentally demonstrated to increase in the orderchloride<bromide<iodide. In the Pt(II)TMPyP porphyrin, the centralcavity is filled with Pt(II) metal and that makes it interestingcandidate as a senor for the detection of halides. A previous study withmetal-free porphyrin dissolved in organic solvent reported a poorsensitivity towards detection of halide (Ref. 19; Makrogeterotsikly,2008, 50). It is interesting to note that Pt(II) porphyrin demonstratedsimilar sensitivity towards iodide detection in methanol (FIG. 9), andin aqueous phase (FIG. 6). A specific metal filling the central cavityof porphyrin has important role, and when Pt(II) was exchanged withZn(II) sensing function for similarly dilute aqueous solutions ofhalides was almost lost (FIG. 10). It would mean that the Pt metalcenter is the key component of the sensing down to such an extremelydetection limit.

The photoluminescence quenching upon the addition of halides indicatesthe presence of a photoinduced electron transfer event between thehalide donor and the Pt(II)TMPyP acceptor. Non-linear Stern-Volmer plotsfor all of the detected halide ions indicate a static reaction for thequenching process (FIG. 7). Furthermore, the bimolecular quenching rateconstant, k_(q)≈7.8×10¹² M⁻¹s⁻¹ far exceeds the diffusion-controlledlimit (˜3×10¹⁰ M⁻¹s⁻¹) establishing the fact that the quenching is dueto static interaction, and confirmed by recording phosphorescencelifetimes (FIG. 8).

The transient absorption spectra of Pt(II)TMPyP-halides demonstrateshortening in the order; Pt(II)TMPyP alone>chloride>bromide>iodide (FIG.2). And this was confirmed from the kinetic traces for the ground-statebleach recovery time values extracted from the transient absorptionspectra (FIG. 3). This overall fast deactivation of the triplet signalimplies the presence of an extra process involved in deactivation of theexcited state when halides are present compared to the case of freePt(II) porphyrin. Moreover, for the control experiment withZn(II)TMPyP-iodide system, no appreciable changes were observed intransient absorption spectra and corresponding kinetics making aconfirmation that the interaction occurs only with the Pt(II) at themacrocycle center of the porphyrin (FIG. 11A-B).

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt% to about 5 wt%, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. In an embodiment, the term “about” can includetraditional rounding according to significant figures of the numericalvalue. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ toabout ‘y’”.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations, andare set forth only for a clear understanding of the principles of thedisclosure. Many variations and modifications may be made to theabove-described embodiments of the disclosure without departingsubstantially from the spirit and principles of the disclosure. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure.

We claim at least the following:
 1. A method of detecting anions,comprising: contacting a solution including an anion with a Pt(II)porphyrin; irradiating the solution with visible light; measuring aphotoluminescence signal from the Pt(II) porphyrin; and determining theconcentration of the anion; wherein the Pt(II) porphyrin is 5, 10, 15,20-tetra(1-methyl-4-pyridino)-porphyrin Pt(II) tetrachloride, ananalogue thereof, or a derivative thereof.
 2. The method of claim 1,wherein the anion is selected from the group consisting of: a halogenanion, a sulfide anion, and a cyanide anion.
 3. The method of claim 1,wherein the anion is iodide.
 4. The method of claim 1, wherein the anionis present at a level of at least about 30 pmol in the solution.
 5. Themethod of claim 1, further comprising: measuring a UV-Vis signal.
 6. Asensor system, comprising: a structure having a Pt(II) porphyrindisposed on the surface; a compartment including a solution including ananion, wherein the anion and Pt(II) porphyrin interact to quench thephotoluminescence of the Pt(II) porphyrin once the structure is placedin the compartment with the solution; a system for irradiating thesolution with visible light; and a system for measuring a signalselected from a photoluminescence signal, UV-Vis signal, or acombination thereof, wherein a change in the signal relative to a signalwithout the anion present is correlated to the presence of the anion inthe solution.
 7. The system of claim 6, wherein the anion is selectedfrom the group consisting of a halogen anion, a sulfide anion, and acyanide anion.
 8. The system of claim 6, wherein the anion is iodide. 9.The system of claim 6, wherein the anion is present at a level of atleast about 30 pmol in the solution.
 10. The system of claim 6, whereinthe Pt(II) porphyrin is 5, 10, 15,20-tetra(1-methyl-4-pyridino)-porphyrin (Pt(II) tetrachloride, ananalogue thereof, or a derivative thereof.
 11. A method comprising,comprising: placing a structure having a Pt(II) porphyrin disposed onthe surface in a compartment including a solution including an anion,wherein the anion and Pt(II) porphyrin interact to quench thephotoluminescence of the Pt(II) porphyrin; irradiating the solution withvisible light; and measuring a signal selected from a photoluminescencesignal, UV-Vis signal, or a combination thereof, wherein a change in thesignal relative to a signal without the anion present is correlated tothe presence of the anion in the solution.
 12. The method of claim 11,wherein the anion is selected from the group consisting of a halogenanion, a sulfide anion, and a cyanide anion.
 13. The method of claim 11,wherein the anion is iodide.
 14. The method of claim 11, wherein theanion is present at a level of at least about 30 pmol in the solution.15. The method of claim 11, wherein the Pt(II) porphyrin is 5, 10, 15,20-tetra(1-methyl-4-pyridino)-porphyrin (Pt(II) tetrachloride, ananalogue thereof, or a derivative thereof.